Fix some glib docs warnings

Remove some symbols from glib-sections.txt that gtk-doc has no idea
about.

Add proper callback typedefs for GTester (gtk-doc dislikes inline
function types).

Fix some other minor issues.

glib/gutf8.c

@@ -95,7 +95,7 @@
       (Result) |= ((Chars)[(Count)] & 0x3f);				      \
     }
     
-/**
+/*
  * Check whether a Unicode (5.2) char is in a valid range.
  *
  * The first check comes from the Unicode guarantee to never encode